Can I use a mysql trigger to control the update of a field if it has a value?

Mysql trigger problems can I use a trigger to control whether a field will reject update once it has a value. one of the 200 fields in table t1 is f1.

When a record is inserted into table t1, the f1 field must be null.

An update changes the f1 field to the current timestamp.

After that Can you control all update statements for this record? once the statement updates this field value, the update is denied.

Whether to use a trigger or other methods. what should I do if I can use a trigger?>

Reply to discussion (solution)

Create trigger upd_check before update on t1 for each row begin if not isnull f1 then set new. f1 = f1; end if; END;

Thanks to xu da'